Some Enhanced Observed Time Difference (E-OTD) location technology architectures initiate E-OTD measurements in cellular subscriber devices by sending a location request from wireless communications networks to subscriber devices. For example, network requests for location information from subscriber devices are made during E-911 calls initiated by subscriber devices. Network location requests are also made in response to inquiries from subscriber devices for location-based services.
E-OTD location requests from networks typically include a list of assistance cells from which a subscriber device must attempt to obtain timing measurements. The assistance cell list is generally different from the list of neighbor cells monitored by the subscriber device for handover procedures.
Generally, the greater the number of cells on which measurements are made by the subscriber device in response to a location request, the better the accuracy of the location computation. However, the network must provide the subscriber device with a relatively short list of assistance cells, since the subscriber device must respond to the location request within a specified time period. Thus it is desirable for the network to accurately identify assistance cells on which the subscriber device will most likely obtain measurement data. Presently, the network selects the assistance cells based on a rough location estimate of the subscriber device. The rough location estimation is derived generally from timing advance and from sector or cell identification information.
Also, presently, networks must specify a response time without any knowledge of the radio conditions experienced by the subscriber device to which the location request is sent. Propagation delay of the network location request through the network is significant relative to the time period during which the subscriber device must perform the requested timing measurements. Similarly, the time required for the measurement information response to propagate from the subscriber device back to the network location server is not insignificant. The propagation delay generally increases the time required to locate the subscriber device.
Upon receipt of a response from a subscriber device, the network must determine the position of the subscriber device using measurement data from a single response of the subscriber device, since the network accepts only a single subscriber device response to the network location request. Thus the network computes the location of the subscriber device based upon assumptions, which may be inaccurate, and without any indication of E-OTD measurement trends.
The various aspects, features and advantages of the present disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description and the accompanying drawings described below.